1. Field of the Invention
This invention generally relates to strain sensor for sensing a change in strain induced by the presence of infrared radiation, and more particularly to infrared sensors comprising organic materials.
2. Description of Related Art
Infrared sensors have many medical, military, industrial and commercial applications. These IR sensors can be generally broken down into two categories—photonic and thermal. Photonic IR sensors have very narrow bands and require cryocooling for operation which results in high power consumption and very large bulk. Though photon detectors may have fast scanning rates and very high sensitivities, the extra bulk that results from using a cooling apparatus is a serious trade-off in achieving high imaging speeds. A significant market exists for infrared detectors that show comparable performance to photon detectors without using cooling. Thermal detectors form a class of infrared detectors, including pyroelectrics, bolometers, thermistors, thermopiles and Golay cells, and are generally uncooled.
Of the thermal sensors, microbolometers show the most promise, due to their seemingly lower sensitivity to noise. However, both microbolometers and pyroelectrics are electro-resistive devices and thus still quite sensitive to thermal noise resulting in lower sensitivities and resolutions compared to cryocooled photonic IR sensors. Their designs are also not as scalable such that pixel sizes are usually 30 or more microns. Microbolometers also require thermal stabilization to attain competitive sensitivities which requires maintenance of temperatures surrounding the device through thermoelectric coolers and/or extensive and often complex circuitry. The noise equivalent change in temperature (NEDT) of current uncooled thermal devices is around 20 mK and at least around an order higher than state-of-the-art photonic IR sensors. To bring the NEDT of uncooled microbolometers closer to that of photonic devices various structures have been suggested, such as double cantilevers, but the structure scalability is sacrificed reducing array resolution. Also, the non-uniformities of the surface of the devices also contribute to sensing problems.
Background on the Material Properties of Chitin
Chitin is the most common nitrogen bearing polysaccharide, and is second only to cellulose as the most abundant polysaccharide formed in nature. Chitin is a polymer of N-acetyl glucosamine (GlcNAc). It is commonly found in the exoskeletons of crustaceans and insects, in the form of threadlike polymer chains of chitin. The spectrum of chitin shows infrared absorption at the 3, 6, and 9 μm bands, corresponding to the presence of carbon-hydrogen single bonds (C—H), carbon oxygen double bonds (C═O), and carbon-oxygen single bonds (C—O), respectively. Additional absorption at 3 μm is due to nitrogen-hydrogen (N—H) single bonds and oxygen-hydrogen single bonds (O—H). Chitin is highly acetylated, with the percent acetylation varying depending on the source of the chitin. Although many insects and crustaceans contain chitin, it is Melanophila acuminata's IR-sensitive pit organ (which is largely composed of chitin), combined with chitin's peak IR absorption and the insect's peak behavioral response to 3 μm radiation that suggests chitin's involvement in the detection of forest fires. The jewel beetle's chitin-composed pit organ is innervated, allowing transduction of the IR absorption into an electrical signal through an action potential.
Infrared absorption occurs when the frequency of the incident infrared energy matches the vibrational resonance of the bond, and a change in dipole moment occurs during the vibration. The absorbed infrared radiation causes either stretching or bending of the bond. To maximize the photomechanical response, the stretching of the bond should translate either throughout the length of a polymer, or between parallel chains.
Chitin is not commonly used in semiconductor and MEMS (microelectromechanical system) devices, and many of its material properties have not yet been characterized. However, the infrared response of chitin makes it useful as a sensory material. See Table 1. Deposition of chitin into a thin film is difficult because it is not water soluble. Chitosan is the deacetylated form of chitin. Chitosan is soluble in water, while chitin is not. Chitosan is soluble in acidic solutions and insoluble in basic solutions. Chitosan may be more easily deposited than chitin due to its net positive charge and solubility in water and acidic solution, which allows it to be electrodeposited.